Dielectric ceramic materials with insulated boundaries between crystal grains, and process for preparation

ABSTRACT

A dielectric ceramic material composed of primary and secondary ingredients forming in combination a polycrystalline ceramic proper, and of insulating substances diffused throughout the intergranular boundaries of the ceramic proper for an increase in apparent relative dielectric constant. The primary ingredients comprise, in relative porportions, 94.50-99.62 wt. % SrTiO3, 0.33-3.00 wt. % Ta2O5, and 0.05-2.50 wt. % ZnO. The secondary ingredients comprise 0.02-0.10 wt. part SiO2 and 0.01-0.03 wt. part Al2O3 with respect to 100 wt. parts of the primary ingredients, with the weight ratio of SiO2 to Al2O3 being from 1.5 to 5.0. The insulating substances comprise 0.03-2.80 wt. % PbO, 0.11-4.38 wt. % Bi2O3, and 0.001-0.18 wt. % B2O3 with respect to the total weight of the primary and the secondary ingredients. In the fabrication of bodies of the ceramic the mixture of the insulating substances is coated or otherwise layered on polycrystalline ceramic bodies prepared from the primary and secondary ingredients. The coated ceramic bodies are then heated to cause diffusion of the insulating substances throughout their intergranular boundaries.

BACKGROUND OF THE INVENTION

Our invention relates to ceramic materials in general and, inparticular, to those composed principally of strontium titanate (SrTiO₃)and especially well suited for use as dielectrics of capacitors. Stillmore particularly, our invention pertains to ceramic materials ofpolycrystalline structure having electrically insulated boundariesbetween the crystal grains with a view to an increase in apparentrelative dielectric constant. The invention also particularly concerns aprocess for the fabrication of coherent bodies of such ceramicmaterials.

The polycrystalline ceramics having insulated intergranular boundariesare generally referred to as the "boundary layer ceramics" in the art.An example is described and claimed in U.S. Pat. No. 3,933,668 issued toTakahashi et al. on Jan. 20, 1976. The boundary layer ceramic accordingto this U.S. patent consists essentially of a major proportion ofSrTiO₃, a minor proportion of niobium oxide (Nb₂ O₅) or tantalum oxide(Ta₂ O₅), and a minor proportion of germanium dioxide (GeO₂) or zincoxide (ZnO). The polycrystalline ceramic of this fundamental compositionhas further diffused therein bismuth trioxide (Bi₂ O₃) or its admixturewith lead monoxide (PbO) and boron oxide (B₂ O₃) for the insulation ofthe boundaries between its crystal grains.

An objection to this known boundary layer ceramic is that its crystalgrains average in diameter only from about 40 to 60 micrometers. Itsapparent relative dielectric constant is therefore up to 75,000 or so.Although this value is satisfactory for most intended applications ofthe material, there are strong demands today for ceramics with stillhigher relative dielectric constants.

SUMMARY OF THE INVENTION

Our invention meets the present day demands for ceramics with suchextremely high relative dielectric constants by making the crystalgrains of boundary layer ceramics much greater in size than in the priorart.

The invention may be summarized as a boundary layer ceramic havinginsulated boundaries between its constituent crystal grains, composed ofprimary and secondary ingredients forming in combination apolycrystalline ceramic proper, and insulating substances dispersed inthe intergranular boundaries of the ceramic proper. The primaryingredients comprise approximately 94.50 to 99.62 percent by weightSrTiO₃, approximately 0.33 to 3.00 percent by weight Ta₂ O₅, andapproximately 0.05 to 2.50 percent by weight ZnO, the percentages ofthese primary ingredients being relative to each other. The secondaryingredients comprise approximately 0.02 to 0.10 part by weight silica(SiO₂) and approximately 0.01 to 0.03 part by weight aluminum oxide (Al₂O₃) with respect to 100 parts by weight of the listed primaryingredients, the ratio in weight of SiO₂ to Al₂ O₃ being approximatelyfrom 1.5 to 5.0. The insulating substances comprise approximately 0.03to 2.80 percent by weight PbO, approximately 0.11 to 4.38 percent byweight Bi₂ O₃, and approximately 0.001 to 0.18 percent by weight B₂ O₃.The percentages of the insulating substances are all with respect to thetotal weight of the noted primary and secondary ingredients in use.

Of the above ingredients of the boundary layer ceramics in accordancewith our invention, the tantalum oxide (Ta₂ O₅) functions principally tomake the ceramics semiconductive. The zinc oxide (ZnO) is effectiveprimarily to accelerate the growth of the crystal grains.

Used in the above specified ranges of proportions, the secondaryingredients SiO₂ and Al₂ O₃ both serve to make the crystal grains of theceramics greater in size than hitherto, namely from 60 to 120micrometers in diameter. Generally, the greater the crystal grains ofceramics in size, the higher are their relative dielectric constants.The apparent relative dielectric constants of the ceramics in accordancewith our invention are well over 80,000. As has been ascertained byexperiment, moreover, the dielectric loss tangents (the tangents of thedielectric loss angles) of the ceramics in accordance with our inventionare less than one percent (1×10⁻²), and their resistivities are over1.0×10¹¹ ohm-centimeters. The ceramics with these characteristics areadmirably well suited for use as dielectrics of capacitors.

It will of course be understood that the listed ingredients of theceramics in accordance with our invention perform not just the aboverecited functions by themselves. They intricately interact to make upthe ceramics of the highly favorable electrical properties.

Our invention also provides a method of fabricating coherent bodies ofthe above ceramic compositions. The method dictates the preparation,first of all, of uninsulated ceramic bodies composed of the primary andsecondary ingredients set forth previously. Then the three insulatingsubstances, PbO, Bi₂ O₃ and B₂ O₃, are suitably layered on theuninsulated ceramic bodies, which are then heated to cause diffusion ofthe insulating substances throughout the intergranular boundaries of theceramic bodies.

Preferably, the uninsulated ceramic bodies are prepared by firingmoldings of the primary and secondary ingredients in a reductiveatmosphere. On these uninsulated ceramic bodies are then coated a pasteof the insulating substances. Then the coated bodies are heated in anoxidative atmosphere to cause diffusion of the insulating substances.The use of a rubber-lined bowl mill is recommended for blending togetherthe primary and secondary ingredients as it makes possible the accurateproportioning of SiO₃ and Al₂ O₃, which is essential to increase thesize of the crystal grains to a desired degree.

The above and other features and advantages of this invention and themanner of attaining them will become more apparent, and the inventionitself will best be understood, from a study of the followingdescription taken together with the attached drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly diagrammatic section through a body of insulatedboundary layer ceramic material in accordance with our invention, theceramic body being shown electroded for use as a capacitor; and

FIG. 2 is a graphic representation of the temperature dependencies ofthe relative dielectric constants of test pieces of various ceramiccompositions in accordance with our invention.

DETAILED DESCRIPTION

Although the showing of FIG. 1 is highly diagrammatic, it willnevertheless be seen that the boundary layer ceramic body 10 of ourinvention is of polycrystalline structure, composed of an aggregate ofsemiconductive crystal grains 11 isolated from one another by boundarylayers 12. Diffused throughout these boundary layers are PbO, Bi₂ O₃ andB₂ O₃ which serve to insulate the crystal grains 11 from one another.The ceramic body 10 can be converted into a capacitor by beingelectroded as at 13. These electrodes may be formed by coating acommercial silver paste on the opposite faces of the ceramic body 10 andthen by firing the coatings.

Presented hereinbelow are Examples dealing with specific compositions ofthe insulated boundary layer ceramics, method of their preparation, andelectrical properties of capacitors made with use of the ceramics. Aswill be specifically pointed out in the course of the description ofthese Examples, some of them fall outside the purview of our invention.Such noninventive Examples are given to clearly distinguish theadvantages of our invention and to give support to the claimed numericallimitations.

Hereinafter in this specification all percentages and parts are byweight unless otherwise specified.

EXAMPLES 1-34

SrTiO₃ of the industrial grade (containing practically negligibleamounts of impurities such as barium, iron, manganese, calcium, sodiumand potassium) was admixed with two other primary ingredients Ta₂ O₅ andZnO and the secondary ingredients SiO₂ and Al₂ O₃ in various sets ofproportions specified in Table 1. In order to avoid intrusion of foreignmatter, these admixtures were blended wet in a rubber-lined bowl mill.After being dried, the admixtures were pasted with polyvinyl alcohol, anorganic binder, and the pastes were formed into test discs. Then thetest discs were heated to and held at 1000° C. for one hour therebyremoving the binder therefrom by evaporation. Then the test discs werefired in a temperature range of 1350° C. to 1450° C. for two to fourhours in a weakly reductive atmosphere of 99 percent by capacity ofmolecular nitrogen (N₂) and one percent by capacity of molecularhydrogen (H₂). Thus were prepared the semiconductive ceramic test discseach having a diameter of approximately eight millimeters and athickness of approximately 0.4 millimeter.

The test discs are polycrystalline, comprising a myriad of crystalgrains 11 as in FIG. 1. These crystal grains are, however, not yetinsulated from one another, so that we will refer to the discs at thisstage as the uninsulated ceramic test discs. It will be appreciated thatthe use of the rubber-lined bowl mill serves to prevent the introductionof impurities, as from a ceramic-made bowl, into the ceramic materials,making possible the precise proportioning of SiO₂ and Al₂ O₃ which areboth used in extremely small quantities.

Then, in order to insulate the crystal grains of the uninsulated ceramictest discs from each other, there was prepared a mixture of 50 percentPbO, 45 percent Bi₂ O₃, and five percent B₂ O₃, all in finely dividedform. The mixture was pasted with nitrocellulose and butyl "Carbitol"(trademark for diethylene glycol mono-n-butyl ether). This paste wasscreen-printed on one face of each uninsulated ceramic test disc, at arate of 10 milligrams (10 percent) for each 100 milligrams of theuninsulated ceramic test discs.

Then the coated test discs were heated to and held at a constanttemperature in the range from 1150° C. to 1300° C. for two hours in aoxidizing atmosphere. This heat treatment caused diffusion of PbO, Bi₂O₃ and B₂ O₃ throughout the intergranular boundaries of the ceramic testdiscs. The insulated boundary layers thus formed are indicated at 12 inFIG. 1. Thus was completed the fabrication of the insulated boundarylayer ceramic test discs in accordance with our invention.

For testing the electrical properties of the test discs, these werefurther processed into test capacitors by coating a commercial silverpaste on their opposite faces and by firing them. FIG. 1 shows at 13 thepair of electrodes thus formed on each test disc.

In the foregoing process of the fabrication of the insulated boundarylayer ceramic test discs, the secondary ingredients SiO₂ and Al₂ O₃ areboth wholly retained in the completed products in the initialproportions. However, the insulating substances PbO, Bi₂ O₃ and B₂ O₃partly evaporate on being heated after having been coated on theuninsulated ceramic test discs, the remainders of the substances beingdiffused in the intergranular boundaries of the test discs. Theremaining (actually diffused) percentages of the insulating substancesare subject to change depending upon the substances themselves and onthe compositions of the test discs. In these particular Examples the PbOremained at rates ranging from 0.27 to 0.79 milligram (0.27 to 0.79percent), the Bi₂ O₃ at rates ranging from 0.51 to 1.50 milligrams (0.51to 1.50 percent), and the B₂ O₃ at rates ranging from 0.01 to 0.05milligram (0.01 to 0.05 percent), for each 100 milligrams of the testdiscs.

The boundary layer ceramic test capacitors prepared as above were testedas to apparent relative dielectric constants (ε), dielectric losstangents (tan δ), and resistivities (ρ). Table 1 gives the results. Inthis and all the following tables each listed value is the average of 30samples. Also, in all the tables given herein, relative dielectricconstants and dielectric loss tangents were both measured at onekilo-hertz, and resistivities were measured upon lapse of one minuteafter application of a direct current at 50 volts. The dielectric losstangents in the tables are expressed in percent; therefore, for theactual values, the tabulated figures should be multiplied by 10⁻².

It will also be noted from Table 1 that the percentages of the threeprimary ingredients, SrTiO₃, Ta₂ O₅ and ZnO, sum up to 100 in eachExample. The same holds true with the various combinations of threeprincipal ingredients employed in all the Examples given in thesubsequent tables. Further, in this and all the following tables, theproportions of the secondary ingredients SiO₂ and Al₂ O₃ are expressedin parts with respect to 100 parts of the three primary ingredientscombined.

                                      TABLE 1                                     __________________________________________________________________________    Primary ingredients                                                                            Secondary ingredients                                        (100 wt. parts)  SiO.sub.2,                                                                       Al.sub.2 O.sub.3,                                                                      Electrical properties                             plesExam-                                                                         wt. %SrTiO.sub.2,                                                                  wt. %Ta.sub.2 O.sub.5,                                                            wt. %ZnO,                                                                         partwt.                                                                          partwt.                                                                           ##STR1##                                                                           × 10.sup.4ε,                                                        %tan δ,                                                                     × 10.sup.11 Ω-cmρ,      __________________________________________________________________________     1  98.50                                                                              1.20                                                                              0.30                                                                              -- --  --   6.7 0.6   2.0                                     2  94.50                                                                              3.00                                                                              2.50                                                                              0.024                                                                            0.028                                                                             0.9  6.0 0.8   2.0                                     3  99.62                                                                              0.33                                                                              0.05                                                                              0.020                                                                            0.020                                                                             1.0  6.3 0.7   2.0                                     4  98.14                                                                              1.80                                                                              0.06                                                                              0.040                                                                            0.030                                                                             1.3  6.5 0.8   2.0                                     5  97.17                                                                              0.33                                                                              2.50                                                                              0.024                                                                            0.018                                                                             1.3  6.1 0.6   2.0                                     6  96.95                                                                              3.00                                                                              0.05                                                                              0.034                                                                            0.025                                                                             1.4  6.3 0.6   2.0                                     7  96.25                                                                              1.75                                                                              2.00                                                                              0.015                                                                            0.010                                                                             1.5  6.4 0.6   2.0                                     8  98.50                                                                              1.20                                                                              0.30                                                                              0.021                                                                            0.014                                                                             1.5  9.2 0.5   1.7                                     9  97.17                                                                              0.33                                                                              2.50                                                                              0.030                                                                            0.020                                                                             1.5  9.1 0.6   1.7                                    10  94.50                                                                              3.00                                                                              2.50                                                                              0.045                                                                            0.030                                                                             1.5  8.8 0.6   1.8                                    11  99.40                                                                              0.50                                                                              0.10                                                                              0.040                                                                            0.025                                                                             1.6  9.9 0.6   1.6                                    12  99.62                                                                              0.33                                                                              0.05                                                                              0.060                                                                            0.035                                                                             1.7  6.3 0.8   2.0                                    13  96.25                                                                              1.75                                                                              2.00                                                                              0.052                                                                            0.028                                                                             1.9  9.8 0.6   1.6                                    14  99.62                                                                              0.33                                                                              0.05                                                                              0.020                                                                            0.010                                                                             2.0  8.7 0.5   1.8                                    15  96.95                                                                              3.00                                                                              0.05                                                                              0.043                                                                            0.020                                                                             2.2  9.7 0.5   1.7                                    16  97.80                                                                              0.70                                                                              1.50                                                                              0.070                                                                            0.030                                                                             2.3  10.2                                                                              0.6   1.6                                    17  98.14                                                                              1.80                                                                              0.06                                                                              0.078                                                                            0.031                                                                             2.5  6.5 0.6   2.0                                    18  94.50                                                                              3.00                                                                              2.50                                                                              0.032                                                                            0.012                                                                             2.7  8.4 0.5   1.8                                    19  97.80                                                                              0.70                                                                              1.50                                                                              0.095                                                                            0.035                                                                             2.7  6.5 0.7   2.0                                    20  98.50                                                                              1.20                                                                              0.30                                                                              0.060                                                                            0.020                                                                             3.0  11.3                                                                              0.5   1.5                                    21  96.25                                                                              1.75                                                                              2.00                                                                              0.090                                                                            0.028                                                                             3.2  10.2                                                                              0.5   1.6                                    22  97.17                                                                              0.33                                                                              2.50                                                                              0.100                                                                            0.030                                                                             3.3  8.7 0.6   1.8                                    23  98.14                                                                              1.80                                                                              0.06                                                                              0.102                                                                            0.026                                                                             3.9  6.5 0.6   2.0                                    24  98.50                                                                              1.20                                                                              0.30                                                                              0.020                                                                            0.005                                                                             4.0  6.8 0.6   2.0                                    25  98.14                                                                              1.80                                                                              0.06                                                                              0.040                                                                            0.010                                                                             4.0  10.2                                                                              0.5   1.7                                    26  96.95                                                                              3.00                                                                              0.05                                                                              0.080                                                                            0.020                                                                             4.0  10.2                                                                              0.6   1.6                                    27  99.62                                                                              0.33                                                                              0.05                                                                              0.070                                                                            0.015                                                                             4.7  13.1                                                                              0.6   1.2                                    28  98.50                                                                              1.20                                                                              0.30                                                                              0.040                                                                            0.008                                                                             5.0  6.8 0.6   2.0                                    29  97.80                                                                              0.70                                                                              1.50                                                                              0.050                                                                            0.010                                                                             5.0  14.3                                                                              0.5   1.1                                    30  99.40                                                                              0.50                                                                              0.10                                                                              0.100                                                                            0.020                                                                             5.0  9.8 0.6   1.6                                    31  96.25                                                                              1.75                                                                              2.00                                                                              0.080                                                                            0.015                                                                             5.3  14.8                                                                              1.3   0.1                                    32  98.14                                                                              1.80                                                                              0.06                                                                              0.055                                                                            0.010                                                                             5.5  15.6                                                                              1.5   0.1                                    33  97.17                                                                              0.33                                                                              2.50                                                                              0.090                                                                            0.015                                                                             6.0  10.3                                                                              1.1   0.9                                    34  96.95                                                                              3.00                                                                              0.05                                                                              0.095                                                                            0.012                                                                             7.9  8.2 1.0   0.8                                    __________________________________________________________________________

Of the thirty-four Examples of Table 1, Examples 8-11, 13-16, 18, 20-22,25-27, 29 and 30 represent ceramic compositions complying with theteachings of our invention. The relative proportions of the threeprimary ingredients in use, SrTiO₃, Ta₂ O₅ and ZnO, of these Examplesare from 94.50 to 99.62 percent, from 0.33 to 3.00 percent, and from0.05 to 2.50 percent, respectively. Their SiO₂ and Al₂ O₃ contents arefrom 0.02 to 0.10 part, and from 0.01 to 0.03 part, respectively, withrespect to 100 parts of the primary ingredients. The ratios in weight ofSiO₂ to Al₂ O₃ are from 1.5 to 5.0.

The crystal grains of the ceramic test discs of the above inventiveExamples averaged from 60 to 120 micrometers in diameter. As is seenfrom Table 1, the boundary layer ceramic test capacitors of theinventive Examples have apparent relative dielectric constants rangingfrom 84,000 to as high as 143,000, dielectric loss tangents of as smallas 0.5 to 0.6 percent, and resistivities of 1.1 to 1.8×10¹¹ohm-centimeters.

The ceramic compositions of Examples 1-7, 12, 17, 19, 23, 24, 28 and31-34, on the other hand, do not conform to the principles of ourinvention. A consideration of Table 1 will show that if the weight ratioof SiO₂ to Al₂ O₃ is not more than 1.5, the apparent relative dielectricconstants of the resulting capacitors fall short of 80,000. If thisweight ratio exceeds 5.0, on the other hand, then the resultingcapacitors have dielectric loss tangents of over 1.0 percent orresistivities of less than 1.0×10¹¹ ohm-centimeters. Also, if theproportion of SiO₂ is less than about 0.02 part or more than about 0.10part with respect to 100 parts of the primary ingredients, the apparentrelative dielectric constants of the resulting capacitors become lessthan 80,000. The apparent relative dielectric constants do not exceed80,000, either, if the proportion of Al₂ O₃ is less than about 0.01 partor more than about 0.03 part.

We conclude from the above tabulated results that, for the boundarylayer ceramics composed principally of SrTiO₃, Ta₂ O₅ and ZnO inaccordance with our invention, the contents of the secondary ingredientsSiO₂ and Al₂ O₃ should be from about 0.02 to about 0.10 part, and fromabout 0.01 to about 0.03 part, respectively, with respect to 100 partsof the primary ingredients. Further the weight ratio of SiO₂ to Al₂ O₃should be from about 1.5 to about 5.0. Experiment has proved thatcapacitors of no favorable electrical properties are obtainable fromboundary layer ceramics whose proportions of the three primaryingredients fall outside the ranges of our invention, even if their SiO₂and Al₂ O₃ contents and the weight ratios thereof are within the rangesof our invention.

We measured the temperature dependencies of the apparent relativedielectric constants of the above inventive Examples. The results wereas graphically represented in FIG. 2. With the apparent relativedielectric constants at 20° C. taken as a criterion, the percentvariations of the dielectric constants of all the tested Examples withtemperatures fell within the hatched regions of the graph. That is, thepercent variations of the dielectric constants were plus or minus 15 inthe temperature range of -25° C. to +85° C.

EXAMPLES 35-68

Uninsulated ceramic test discs were fabricated by use of the sameingredients, and in the same proportions, as in Examples 10, 14, 16, 20,22, 26, 29 and 30, and through the same procedure as in Examples 1-34.Then the three insulating substances, PbO, Bi₂ O₃ and B₂ O₃, in thevarious sets of proportions given in Table 2 were thermally diffusedthroughout the intergranular boundaries of the test discs through thesame procedure as in Examples 1-34. The thus insulated boundary layerceramic test discs were then processed into test capacitors through thesame procedure as in Examples 1-34.

Measured by the same methods as in Examples 1-34, the three electricalproperties in question of the above prepared test capacitors were asgiven in Table 2. In this table the column labelled "Ceramic"represents, of course, the uninsulated ceramics composed only of theprimary ingredients SrTiO₃, Ta₂ O₅ and ZnO and the secondary ingredientsSiO₂ and Al₂ O₃.

                  TABLE 2                                                         ______________________________________                                                             Electrical                                                                    properties                                               Ex-            Insulating substances tan ρ,                               am-            PbO,    Bi.sub.2 O.sub.3,                                                                   B.sub.2 O.sub.3,                                                                    ε,                                                                           δ,                                                                          × 10.sup.11               ple  Ceramic   wt. %   wt. % wt. % × 10.sup.4                                                                    %   Ω-cm                       ______________________________________                                             Same as:                                                                 35   Example 10                                                                              30      60    10    7.8   0.7 1.9                              36   Example 10                                                                              34      64    2     8.1   0.7 1.9                              37   Example 10                                                                              45      45    10    8.6   0.6 1.8                              38   Example 10                                                                              56      42    2     8.8   0.6 1.8                              39   Example 14                                                                              31      66    3     8.0   0.8 1.9                              40   Example 14                                                                              39      52    9     8.4   0.5 1.8                              41   Example 14                                                                              54      45    1     8.1   0.6 0.3                              42   Example 14                                                                              54      40    6     8.2   0.5 1.8                              43   Example 16                                                                              34      56    10    9.5   0.6 1.6                              44   Example 16                                                                              47      52    1     9.0   0.6 0.4                              45   Example 16                                                                              54      36    10    10.7  0.6 1.6                              46   Example 16                                                                              57      41    2     10.4  0.7 0.6                              47   Example 20                                                                              31      59    10    10.3  0.7 1.6                              48   Example 20                                                                              40      58    2     10.5  0.7 1.6                              49   Example 20                                                                              40      49    11    9.5   1.2 1.5                              50   Example 20                                                                              54      44    2     11.1  0.5 1.5                              51   Example 20                                                                              57      33    10    12.1  0.6 0.5                              52   Example 22                                                                              31      67    2     8.0   1.1 1.9                              53   Example 22                                                                              35      58    7     8.1   0.7 1.9                              54   Example 22                                                                              45      52    3     8.0   0.7 1.8                              55   Example 22                                                                              56      34    10    9.1   0.6 1.8                              56   Example 26                                                                              32      66    2     9.2   0.7 1.7                              57   Example 26                                                                              45      47    8     9.8   0.6 1.6                              58   Example 26                                                                              50      40    10    10.4  0.6 1.6                              59   Example 26                                                                              55      40    5     10.0  0.6 1.6                              60   Example 29                                                                              32      64    4     12.7  0.7 1.3                              61   Example 29                                                                              38      57    5     13.3  0.6 1.2                              62   Example 29                                                                              50      38    12    15.4  1.3 1.0                              63   Example 29                                                                              53      38    9     14.2  0.6 1.1                              64   Example 30                                                                              31      62    7     8.8   0.7 1.7                              65   Example 30                                                                              35      55    10    9.3   0.6 1.6                              66   Example 30                                                                              45      52    3     9.1   0.6 1.6                              67   Example 30                                                                              56      42    2     9.8   0.6 1.6                              68   Example 30                                                                              56      34    10    10.3  0.6 1.6                              ______________________________________                                    

In Examples 36-40, 42, 43, 45, 47, 48, 50, 53-61, and 63-68 of Table 2the relative proportions of the insulating substances PbO, Bi₂ O₃ and B₂O₃ are in the ranges from 31 to 56 percent, from 34 to 66 percent, andfrom two to 10 percent, respectively. The apparent relative dielectricconstants of all these inventive Examples are sufficiently high, andtheir dielectric loss tangents and resistivities are also favorable. InExamples 35, 41, 44, 46, 49, 51, 52 and 62, on the other hand, therelative proportions of PbO, Bi₂ O₃ and B₂ O₃ all fall outside the aboveranges. All these noninventive Examples are unsatisfactory in one ormore of the three electrical properties.

We conclude from the results of Table 2 that the ranges of the relativeproportions of the three insulating substances PbO, Bi₂ O₃ and B₂ O₃should be from about 31 to about 56 percent, from about 34 to about 66percent, and from about two to about 10 percent, respectively, for theceramic compositions now under consideration.

We measured the percentages of the insulating substances remainingdiffused in the ceramic test discs of the above inventive Examples afterthe heat treatment. PbO remained at rates ranging from 0.06 to 1.79percent Bi₂ O₃ at rates ranging from 0.23 to 2.80 percent, and B₂ O₃ atrates ranging from 0.002 to 0.12 percent.

EXAMPLE 69

In this Example it was intended to ascertain the percentages of thethree insulating substances to remain diffused in the boundary layerceramics composed principally of SrTiO₃, Ta₂ O₅ and ZnO, for theprovision of capacitors of favorable properties. The relativeproportions of the insulating substances in use were within the rangesspecified in connection with Examples 35-68. The pasted mixture of theinsulating substances was coated on uninsulated ceramic test discs(prepared through the same procedure as in Examples 1-34, with theproportions of the ingredients in conformity with our invention) invarious amounts ranging from one to 15 percent of the weight of the testdiscs. The coated discs were heated at various temperatures ranging from1150° C. to 1300° C. for various lengths of time ranging from one tofour hours, thereby preparing a number of insulated boundary layerceramic test discs containing various amounts of the insulatingsubstances. Then the test discs were processed into test capacitorsthrough the same procedure as in Examples 1-34.

Of the thus fabricated test capacitors, those having an apparentrelative dielectric constant of over 80,000, a dielectric loss tangentof not more than one percent, and a resistivity of not less than1.0×10¹¹ ohm-centimeters were picked up. The PbO, Bi₂ O₃ and B₂ O₃contents of these test capacitors with the favorable electricalproperties ranged from 0.03 to 2.80 percent, from 0.11 to 4.38 percent,and from 0.001 to 0.18 percent, respectively, with respect to the weightof the uninsulated ceramic discs (i.e., those composed of the primaryingredients SrTiO₃, Ta₂ O₅ and ZnO and secondary ingredients SiO₂ andAl₂ O₃).

Although we have disclosed the boundary layer ceramics of our inventionand the method of their preparation in terms of specific Examples, weunderstand that these are meant purely to illustrate or explain and notto impose limitations upon the invention, since a variety ofmodifications or variations will readily occur to the ceramicsspecialist on the basis of this disclosure. The following is a brieflist of such modifications and variations:

1. The insulating substances PbO, Bi₂ O₃ and B₂ O₃ may not necessarilybe used as starting substances in the manufacture of the boundary layerceramics in accordance with our invention, all that is required beingthat they exist as such in the intergranular boundaries of the completedceramics. Other substances may therefore be adopted as well, only ifthey become PbO, Bi₂ O₃ and B₂ O₃ on being heated for diffusion in theintergranular boundaries. Examples include red lead oxide (Pb₃ O₄), leadfluoride (PbF₂), lead borate (Pb(BO₂)₂.H₂ O), and bismuth trifluoride(BiF₃).

2. As another method of diffusing the insulating substances in theintergranular boundaries, there may be prepared a mixture of substancesthat will be thermally converted into PbO, Bi₂ O₃ and B₂ O₃ in thedesired ranges of proportions. This mixture may be fired at, forinstance, 1000° C. to provide the required insulating substances, thenpulverized, pasted, and coated on uninsulated ceramic bodies.

3. The insulating substances may not necessarily be coated onuninsulated ceramic bodies but may be otherwise layered thereon as byvacuum deposition or by immersion of the bodies therein.

4. The primary ingredients SrTiO₃, Ta₂ O₅ and ZnO and the auxiliaryingredients SiO₂ and Al₂ O₃ are also required to exist as such only inthe completed ceramics; starting substances may be different. Thus, forexample, strontium carbonate and titanium oxide may be used as startingsubstances to provide SrTiO₃ in the completed ceramics.

Additional modifications and variations of our invention may be resortedto without departing from the scope of the appended claims.

We claim:
 1. An insulated boundary layer ceramic having primary andsecondary ingredients forming in combination a polycrystalline ceramicproper, and insulating substances dispersed throughout the intergranularboundaries of the ceramic proper, the primary ingredients consistingessentially of, in relative proportions, from about 94.50 to about 99.62percent by weight SrTiO₃, from about 0.33 to about 3.00 percent byweight Ta₂ O₅, and from about 0.05 to about 2.50 percent by weight ZnO,the secondary ingredients consisting essentially of from about 0.02 toabout 0.10 part by weight SiO₂ and from about 0.01 to about 0.03 part byweight Al₂ O₃ with respect to 100 parts by weight of the primaryingredients, the ratio in weight of SiO₂ to Al₂ O₃ being from about 1.5to about 5.0, and the insulating substances consisting essentially offrom about 0.03 to about 2.8 percent by weight PbO, from about 0.11 toabout 4.38 percent by weight Bi₂ O₃, and from about 0.001 to about 0.18percent by weight B₂ O₃, the percentages of the insulating substancesbeing all with respect to the total weight of the primary and thesecondary ingredients.